Silicon dioxide film forming method

ABSTRACT

This invention is an oxynitride film forming method including: a reaction chamber heating step of heating a reaction chamber to a predetermined temperature, the reaction chamber containing an object to be processed; a gas heating step of heating a process gas to a temperature not lower than a reaction temperature at which an oxynitride film can be formed, the process gas consisting of dinitrogen oxide gas; and a film forming step of forming an oxynitride film on the object to be processed by supplying the heated process gas into the heated processing chamber. The temperature to which the reaction chamber is heated in the reaction chamber heating step is set at a temperature below a temperature at which the process gas undergoes a reaction.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a method of forming anoxynitride film or the like and a system for carrying out the same. Morespecifically, the present invention relates to a method of forming anoxynitride film or the like on an object to be processed, such as asemiconductor wafer, and a system for carrying out the same.

[0003] 2. Description of the Related Art

[0004] A semiconductor device fabricating process forms an insulatingfilm on an object to be processed, such as a semiconductor wafer. Thisinsulating film is used, for example, as a mask for impurity diffusionor ion implantation or as a source of an impurity. A silicon oxynitridefilm is used occasionally as such an insulating film. Silicon oxynitridefilms, as compared with prevalently used silicon oxide films, have ahigh dielectric constant and have a high capability of preventingpenetration by an impurity, such as boron.

[0005] A silicon oxynitride film is formed on a surface of asemiconductor wafer by, for example, subjecting a semiconductor wafer toa thermal process. This thermal process will be described. Asemiconductor wafer, such as a silicon wafer, is placed in a thermalprocessing device. The semiconductor wafer is heated to a hightemperature of, for example, 900° C. Then, a process gas, such asdinitrogen oxide gas (N₂O gas) is supplied into the thermal processingdevice for a predetermined time to form a silicon oxynitride film on thesurface of the semiconductor wafer.

[0006] The progressive miniaturization of semiconductor devices requiresreduction of thickness of silicon oxynitride films. Generally, it ispreferable to lower the process temperature in the thermal processingdevice to form silicon oxynitride films of a small thickness, becausethe lowering of the process temperature is effective in reducingoxidation rate.

[0007] However, if the process temperature is lowered, for example, from900° C. to 800° C. or 750° C., nitrogen gas cannot satisfactorily bepyrolyzed and, consequently, it is difficult to form an oxynitride filmhaving a desired nitrogen content.

[0008] Methods of forming a silicon oxide film (SiO₂ film) on each of aplurality of semiconductor wafers (hereinafter referred to simply as“wafers”) placed in a batch-processing furnace by oxidizing a siliconfilm on each wafer are classified into: dry oxidation methods that useoxygen gas (O₂ gas) and hydrogen chloride gas (HCl gas); and wetoxidation methods that produce steam by burning oxygen gas and hydrogengas (H₂ gas) by an external device and that supply the steam and oxygengas into a reaction tube. A suitable oxidation method is selectedaccording to desired film quality.

[0009] The dry oxidation methods oxidize a silicon film with oxygen gasand remove impurities from the surface of the wafer by means ofgettering-effect of chloride. More concretely, a wafer boat holding aplurality of wafers in a tier-like manner is carried into a verticalreaction tube, a process atmosphere in the reaction tube is heated by aheater surrounding the reaction tube, a process gas of an ordinarytemperature including oxygen gas and hydrogen chloride gas is suppliedthrough a ceiling part of the reaction tube into the reaction tube, andthe process atmosphere is exhausted through a lower part of the reactiontube.

[0010] Higher process temperatures are more apt to produce a defectcalled a slip. In addition, it is preferable to avoid thermallyaffecting underlying films and to reduce energy consumption. Therefore,various studies have been made to reduce process temperature.

[0011] Since a diameter of the wafer is increasing progressively,thickness uniformity of a film formed on the surface of the wafer, i.e.,intrasurface thickness uniformity becomes worse when the processtemperature is reduced. In addition, thickness difference between filmsformed on the surfaces of the wafers, i.e., interwafer thicknessuniformity also becomes worse.

[0012] It has been found, through examinations of relation between theposition of a wafer on a wafer boat and the thickness of a film formedon the same wafer, that the thickness uniformity of films formed onwafers held in an upper part of the wafer boat is worse than that offilms formed on wafers held in a lower part of the wafer boat. Theinventors of the present invention infer that dependence of thicknessuniformity on the position of the wafer on the wafer boat is due to thefollowing reasons. FIGS. 19A to 19C show typically a flow of a gas overa wafer W, a temperature of the wafer W and a thickness of a film formedon the wafer W, respectively. Oxygen gas and hydrogen chloride gas flowfrom a periphery (edge) of the wafer W toward a center of the same.Then, oxygen gas oxidizes silicon on the wafer W as the same flows alongthe surface of the wafer W. Since the wafer W dissipates heat through aperipheral part thereof, the temperature of the wafer W increases towardthe center of the wafer W. High temperature promotes the oxidation, andhence silicon on a central part of the wafer W is oxidized at anoxidation rate higher than that at which silicon on a peripheral part ofthe wafer W is oxidized. Consequently, even if the film is formed in ahighly uniform thickness, there is a tendency for a part of the film ona central part of the wafer W to be thicker than a part of the same on aperipheral part of the wafer W.

[0013] Although it is only a small amount, interaction between hydrogen,which has been produced through decomposition of hydrogen chloride, andoxygen produces steam. The gas around an upper part of the wafer boat isnot heated sufficiently. Thus, the temperature of the gas rises as thesame flows from the periphery toward the center of the wafer W.Consequently, the amount of steam produced around the center of thewafer W is greater than that of steam produced around the periphery ofthe wafer W. The steam is effective in increasing the oxide film. Thus,the difference between the amount of steam produced around theperipheral part of the wafer W and that of steam produced around thecentral part of the wafer W greatly affects the difference between thethickness of a part of the film formed on the peripheral part of thewafer W and that of a part of the film formed on the central part of thewafer W. Consequently, the thickness of the part of the film on thecentral part of the wafer W is further increased so that the thicknessof the film formed on the wafer W has a distribution of an upward convexcurve, that is, the uniformity of the film thickness becomes worse.Since the temperature of the gas increases as the gas flows toward thelower part of the reaction tube, the above steam generating reaction issubstantially equilibrated around the lower part of the wafer boat. Thatis, the gas is decomposed completely and all the possible amount ofsteam is produced before the gas flows along the wafers W. Therefore,substantially the same amount of steam exists around the peripheral partof the wafer W and around the central part of the wafer W as the processgas flows from the periphery toward the center of the wafer W and,consequently, the film is formed in a highly uniform thickness. Thus, itis inferred that the uniformity of the thickness of the films formed onthe wafers held in the upper part of the wafer boat is considerably bad,and the difference between the thickness of the films formed on thewafers held in the upper part of the wafer boat and that of the filmsformed on the wafers held in the lower part of the wafer boat is great.Accordingly, it is difficult to lower the process temperature at thepresent.

[0014] A semiconductor device fabricating apparatus forms a thin siliconnitride film on an object to be processed, such as a semiconductorwafer. The silicon nitride film is excellent in insulating performanceand corrosion resistance, and is used prevalently as an insulating film,as a means for impurity diffusion and as a mask for ion implantation.The silicon nitride film is formed on a semiconductor wafer by, forexample, a CVD process (chemical vapor deposition process).

[0015] When forming a silicon nitride film on a semiconductor wafer,such as a silicon wafer, by the CVD process, the semiconductor wafer isplaced in a thermal processing apparatus. Subsequently, an interior ofthe thermal processing apparatus is evacuated to a predeterminedpressure of, for example, 133 Pa (1 Torr), and is heated to apredetermined temperature in a range of, for example, 650 to 700° C.Then, process gases, such as dichlorosilane gas (SiH₂Cl₂ gas) andammonia gas (NH₃ gas), are supplied into the thermal processingapparatus for a predetermined time in order to deposit a silicon nitridefilm on a surface of the semiconductor wafer.

[0016] The silicon nitride film thus formed has a refractive indexRI=2.0 and has a substantially stoichiometric composition.

[0017] When forming the silicon nitride film, it is desired to use a lowprocess temperature. However, ammonia cannot be satisfactorilydecomposed and the silicon nitride film cannot be satisfactorilydeposited if the process temperature is as low as 600° C., becauseammonia has a high decomposition temperature. The inventors made variousstudies to use trimethylamine (TMA) having a decomposition temperaturelower than that of ammonia, instead of ammonia, as a source of nitrogen.

[0018] A silicon nitride film formed on a semiconductor wafer by using aprocess temperature of for example 550° C. and trimethylamine as asource of nitrogen had an RI=2.9, which proved that the silicon nitridefilm was not satisfactorily nitrided. Such unsatisfactory nitriding isdue to a large heat capacity of trimethylamine and hence difficulty inheating trimethylamine. Trimethylamine has a constant-pressure heatcapacity (constant-pressure molar heat capacity) at 550° C. of 190J/mol·K, which is about four times the constant-pressure heat capacityof 50 J/mol·K of ammonia. Under the above nitriding condition,deposition rate was as low as 0.27 nm/min, which proved that the siliconnitride film forming process using trimethylamine is not suitable formass production.

[0019] A semiconductor device fabricating apparatus forms a silicondioxide film on an object to be processed, such as a semiconductorwafer, by means of a chemical vapor deposition process (CVD process) orthe like.

[0020] When forming a silicon dioxide film on a semiconductor wafer,such as a silicon wafer, by the CVD process, the semiconductor wafer isplaced in a thermal processing device. Subsequently, an interior of thethermal processing device is evacuated to a predetermined pressure in arange of, for example, 13.3 Pa (0.1 Torr) to 1330 Pa (10 Torr), and isheated to a predetermined temperature in a range of, for example, 700 to900° C. Then, process gases, such as dichlorosilane gas (SiH₂Cl₂ gas)and dinitrogen oxide gas (N₂O gas), are supplied into the thermalprocessing device for a predetermined time. Thus, the dichlorosilane isoxidized, and a silicon dioxide film is deposited on a surface of thesemiconductor wafer.

[0021] The silicon dioxide film thus formed is dense, excellent ininsulating performance and resistant to peeling.

[0022] However, when forming the silicon dioxide film on thesemiconductor wafer by the aforesaid chemical vapor deposition process,the silicon dioxide film is deposited on the semiconductor wafer at alow deposition rate.

SUMMARY OF THE INVENTION

[0023] It is a first object of the present invention to provide anoxynitride film forming method and an oxynitride film forming systemcapable of forming a thin oxynitride film having a desired nitrogencontent.

[0024] According to one feature of the present invention, an oxynitridefilm forming method comprises: a reaction chamber heating step ofheating a reaction chamber to a predetermined temperature, the reactionchamber containing an object to be processed; a gas heating step ofheating a process gas to a temperature not lower than a reactiontemperature at which an oxynitride film can be formed, the process gasconsisting of dinitrogen oxide gas; and a film forming step of formingan oxynitride film on the object to be processed by supplying the heatedprocess gas into the heated processing chamber; wherein the temperatureto which the reaction chamber is heated in the reaction chamber heatingstep is set at a temperature below a temperature at which the processgas undergoes a reaction.

[0025] According to the feature, the temperature of the reaction chamberis set below the reaction temperature of the process gas. Consequently,the oxidation rate by the process gas supplied into the reaction chamberis reduced and hence a thin oxynitride film can be formed. On the otherhand, the process gas is heated in advance to a temperature not lowerthan the reaction temperature at which an oxynitride film can be formed,and is then supplied into the reaction chamber in a state suitable foroxynitriding. Consequently, an oxynitride film having a desired nitrogencontent can be formed on the object to be processed.

[0026] Preferably, the process gas is heated to a temperature at whichthe process gas is pyrolyzed substantially completely, in the gasheating step. In the case, the process gas can have a high nitrogenconcentration, and hence an oxynitride film having a desired nitrogencontent can be surely formed on the object to be processed.

[0027] Preferably, the reaction chamber is heated to a temperature in arange of 750 to 850° C. in the reaction chamber heating step, and theprocess gas is heated to 900° C. or above in the gas heating step. Whenthe process gas is heated at 900° C. or above, the process gas ispyrolyzed substantially completely. In addition, a thin oxynitride filmcan be formed when the temperature of the reaction chamber is set in therange of 750 to 850° C.

[0028] According to another feature of the present invention, anoxynitride film forming system comprises: a reaction vessel defining areaction chamber that can contain an object to be processed; a reactionchamber heating unit that can heat the reaction chamber to apredetermined temperature; a process gas supplying unit that can supplya process gas into the reaction chamber, the process gas consisting ofdinitrogen oxide gas; a gas heating unit, provided at the gas supplyingunit, that can heat the process gas to a predetermined temperaturebefore the process gas is supplied into the reaction chamber; and acontroller that can control the gas heating unit so as to heat theprocess gas to a temperature not lower than a reaction temperature atwhich an oxynitride film can be formed and control the reaction chamberheating unit so as to heat the reaction chamber to a temperature below areaction temperature at which the process gas undergoes a reaction.

[0029] According to the feature, by means of the controller, thetemperature of the reaction chamber is controlled below the reactiontemperature of the process gas. Consequently, the oxidation rate by theprocess gas supplied into the reaction chamber is reduced and hence athin oxynitride film can be formed. On the other hand, by means of thecontroller, the process gas is heated in advance to a temperature notlower than the reaction temperature at which an oxynitride film can beformed, and is then supplied into the reaction chamber in a statesuitable for oxynitriding. Consequently, an oxynitride film having adesired nitrogen content can be formed on the object to be processed.

[0030] Preferably, the controller is adapted to control the gas heatingunit to heat the process gas to a temperature at which the process gasis pyrolyzed substantially completely. In the case, the process gas canhave a high nitrogen concentration, and hence an oxynitride film havinga desired nitrogen content can be surely formed on the object to beprocessed.

[0031] Preferably, the controller is adapted to control the gas heatingunit to heat the process gas to 900° C. or above, and to control thereaction chamber heating unit to heat the reaction chamber to atemperature in a range of 750 to 850° C. When the process gas is heatedat 900° C. or above, the process gas is pyrolyzed substantiallycompletely. In addition, a thin oxynitride film can be formed when thetemperature of the reaction chamber is set in the range of 750 to 850°C.

[0032] In addition, preferably, the reaction vessel defining thereaction chamber includes an inner tube that contains the object to beprocessed and an outer tube that surrounds the inner tube, and the gassupplying unit is adapted to supply the process gas into the inner tube.

[0033] Another second object of the present invention is to providetechniques that enable to form an oxide film of a highly uniformthickness on an object to be processed by subjecting the object to a dryoxidation process while using a low process temperature.

[0034] According to one feature of the present invention, a silicondioxide film forming method comprises: a reaction chamber heating stepof heating a reaction chamber to a predetermined temperature, thereaction chamber containing an object to be processed having a surfaceprovided with at least a silicon layer; a gas pretreating step ofenergizing a process gas to produce water, the process gas containing acompound gas including hydrogen and chlorine, and oxygen gas; and a filmforming step of forming a silicon dioxide film by supplying the processgas that has been energized to produce water into the heated reactionchamber to oxidize the silicon layer of the object to be processed.

[0035] Preferably, the water is produced in the gas pretreating step toan extent such that the process gas does not produce water any furtherat the temperature to which the reaction chamber is heated.

[0036] Preferably, the process gas is energized to produce water byheating the process gas, in the gas pretreating step.

[0037] Preferably, the process gas is heated to a temperature that ishigher than the temperature at which the reaction chamber is heated inthe reaction chamber heating step.

[0038] For example, the compound gas including hydrogen and chlorine isa hydrogen chloride gas.

[0039] According to another feature of the present invention, a silicondioxide film forming system comprises: a reaction vessel defining areaction chamber that can contain an object to be processed having asurface provided with at least a silicon layer; a reaction chamberheating unit that can heat the reaction chamber to a predeterminedtemperature; a process gas supplying unit that can supply a process gasinto the reaction chamber, the process gas containing a compound gasincluding hydrogen and chlorine, and oxygen gas; and a gas heating unit,provided at the gas supplying unit, that can heat the process gas toproduce water before the process gas is supplied into the reactionchamber.

[0040] Preferably, the reaction chamber can contain a plurality ofobjects to be processed in a tier-like manner, and the reaction chamberheating unit has a heater surrounding the reaction chamber.

[0041] Preferably, the gas heating unit comprises a heating vesseldefining a heating chamber packed with flow impeding members and aheating element surrounding the heating chamber, and the heating elementincludes a resistance heating member and a ceramic cover sealing theresistance heating member therein.

[0042] For example, the resistance heating member is made of carbon witha high purity. In addition, for example, the ceramic cover is made ofquartz.

[0043] Another object of the present invention is to provide a siliconnitride film forming method and a silicon nitride film forming systemcapable of forming a silicon nitride film of a substantiallystoichiometric composition at a low process temperature at a high filmforming rate.

[0044] According to one feature of the present invention, a siliconnitride film forming method comprises: a reaction chamber heating stepof heating a reaction chamber to a predetermined temperature, thereaction chamber containing an object to be processed having a surfaceprovided with at least a silicon layer; a reaction chamber pressureregulating step of regulating pressure in the reaction chamber to apredetermined pressure; a gas heating step of preheating trimethylamineto a temperature such that the preheated trimethylamine can producenitrogen when heated in the reaction chamber; and a film forming step offorming a silicon nitride film by supplying a process gas including thepreheated trimethylamine and a silane gas into the reaction chamber tonitride the silicon layer of the object to be processed.

[0045] According to the feature, the trimethylamine is used as a sourceof nitrogen, which enables the nitriding process to be conducted at alow process temperature. Since the trimethylamine is supplied into thereaction chamber after being heated to a temperature not lower than atemperature at which the trimethylamine becomes able to supply nitrogenwhen heated in the reaction chamber, the trimethylamine undergoespyrolysis when heated in the reaction chamber and a large amount ofnitrogen can be supplied to the object to be processed. Consequently, asilicon nitride film of a substantially stoichiometric composition canbe formed at a high film forming rate.

[0046] Preferably, the reaction chamber is heated to a temperature in arange of 400 to 650° C. by the reaction chamber heating step, and thetrimethylamine is heated to a temperature in a range of 500 to 700° C.by the gas heating step. When the trimethylamine that has been heated atthe temperature in the range of 500 to 700° C. is supplied into thereaction chamber that has been heated at the temperature in the range of400 to 650° C., the trimethylamine is pyrolyzed substantially completelyin the reaction chamber.

[0047] Preferably, in the gas heating step, the trimethylamine is heatedunder a pressure in a range of 20 to 90 kPa. In the case, thetrimethylamine can be heated efficiently because the pressure in therange of 20 to 90 kPa is higher than that in the reaction chamber.

[0048] According to another feature of the present invention, a siliconnitride film forming system comprises: a reaction vessel defining areaction chamber that can contain an object to be processed having asurface provided with at least a silicon layer; a reaction chamberheating unit that can heat the reaction chamber to a predeterminedtemperature; a reaction chamber pressure regulating unit that canregulate pressure in the reaction chamber to a predetermined pressure; afirst gas supplying unit that supplies a silane gas into the reactionchamber; a second gas supplying unit that supplies trimethylamine gasinto the reaction chamber; a gas heating unit, provided at the secondgas supplying unit, that can preheat the trimethylamine gas to apredetermined temperature; and a controller that can control the gasheating unit so as to preheat the trimethylamine gas to a preheatingtemperature such that the trimethylamine gas preheated to the preheatingtemperature can produce nitrogen when heated in the reaction chamber.

[0049] According to the feature, the trimethylamine is used as a sourceof nitrogen, which enables the nitriding process to be conducted at alow process temperature. Since the trimethylamine is supplied into thereaction chamber after being heated to a temperature not lower than atemperature at which the trimethylamine becomes able to supply nitrogenwhen heated in the reaction chamber, the trimethylamine undergoespyrolysis when heated in the reaction chamber and a large amount ofnitrogen can be supplied to the object to be processed. Consequently, asilicon nitride film of a substantially stoichiometric composition canbe formed at a high film forming rate.

[0050] Preferably, the reaction vessel defining the reaction chamberincludes an inner tube that contains the object to be processed and anouter tube that surrounds the inner tube, the first gas supplying unitis adapted to supply the silane gas into the inner tube, and the secondgas supplying unit is adapted to supply the trimethylamine gas into theinner tube.

[0051] In addition, preferably, the second gas supplying unit has a gassupply pipe connected to the reaction chamber, and the gas supply pipeis provided with a restricting part formed by reducing an insidediameter of the gas supply pipe on a downstream side of the gas heatingunit. In the case, the trimethylamine stays in a part of the gas supplypipe extending through the gas heating unit for a sufficiently long timeand, consequently, the gas heating unit is able to heat thetrimethylamine at an improved heating efficiency.

[0052] In addition, preferably, the controller is adapted to control thegas heating unit to heat the trimethylamine to a temperature in a rangeof 500 to 700° C., and to control the reaction chamber heating unit toheat the reaction chamber to a temperature in a range of 400 to 650° C.When the trimethylamine that has been heated at the temperature in therange of 500 to 700° C. is supplied into the reaction chamber that hasbeen heated at the temperature in the range of 400 to 650° C., thetrimethylamine is pyrolyzed substantially completely in the reactionchamber.

[0053] Preferably, the gas heating unit is adapted to heat thetrimethyl-amine under a pressure in a range of 20 to 90 kPa. In thecase, the trimethylamine can be heated efficiently because the pressurein the range of 20 to 90 kPa is higher than that in the reactionchamber.

[0054] Another object of the present invention is to provide a silicondioxide film forming method and a silicon dioxide film forming systemcapable of forming a silicon dioxide film on an object to be processed,at a high film forming rate.

[0055] According to one feature of the present invention, a silicondioxide film forming method comprises: a reaction chamber heating stepof heating a reaction chamber to a predetermined temperature, thereaction chamber containing an object to be processed having a surfaceprovided with at least a silicon layer; a reaction chamber pressureregulating step of regulating pressure in the reaction chamber to apredetermined pressure; a gas heating step of preheating dinitrogenoxide gas to a temperature not lower than 700° C.; and a film formingstep of forming a silicon dioxide film by supplying a process gasincluding the preheated dinitrogen oxide gas and a silane gas into thereaction chamber to oxidize the silicon layer of the object to beprocessed.

[0056] According to the feature, the dinitrogen oxide is heated to atemperature not lower than 700° C. before being supplied into thereaction chamber. Thus, pyrolysis of the dinitrogen oxide is promoted, alarge amount of oxygen is produced, and hence oxidation of the silanegas in the reaction chamber is promoted. Consequently, a silicon dioxidefilm can be formed on the object to be processed at a high film formingrate.

[0057] Preferably, in the gas heating step, the dinitrogen oxide isheated to a temperature in a range of 750 to 950° C. When the dinitrogenoxide is supplied into the reaction chamber after heated to thetemperature at 750 to 950° C., pyrolysis of the dinitrogen oxide can bepromoted, and a silicon dioxide film can be formed at a furtherincreased film forming rate.

[0058] According to another feature of the present invention, a silicondioxide film forming system comprises: a reaction vessel defining areaction chamber that can contain an object to be processed having asurface provided with at least a silicon layer; a reaction chamberheating unit that can heat the reaction chamber to a predeterminedtemperature; a reaction chamber pressure regulating unit that canregulate pressure in the reaction chamber to a predetermined pressure; afirst gas supplying unit that supplies a silane gas into the reactionchamber; a second gas supplying unit that supplies dinitrogen oxide gasinto the reaction chamber; a gas heating unit, provided at the secondgas supplying unit, that can preheat the dinitrogen oxide gas to apredetermined temperature; and a controller that can control the gasheating unit so as to preheat the dinitrogen oxide gas to a temperaturenot lower than 700° C.

[0059] According to the feature, the dinitrogen oxide is heated by thegas heating unit to a temperature not lower than 700° C. before beingsupplied into the reaction chamber. Thus, pyrolysis of the dinitrogenoxide is promoted, a large amount of oxygen is produced, and henceoxidation of the silane gas in the reaction chamber is promoted.Consequently, a silicon dioxide film can be formed on the object to beprocessed at a high film forming rate.

[0060] Preferably, the reaction vessel defining the reaction chamberincludes an inner tube that contains the object to be processed and anouter tube that surrounds the inner tube, the first gas supplying unitis adapted to supply the silane gas into the inner tube, and the secondgas supplying unit is adapted to supply the dinitrogen oxide gas intothe inner tube.

[0061] In addition, preferably, the second gas supplying unit has a gassupply pipe connected to the reaction chamber, and the gas supply pipeis provided with a restricting part formed by reducing an insidediameter of the gas supply pipe on a downstream side of the gas heatingunit. In the case, the dinitrogen oxide stays in a part of the gassupply pipe extending through the gas heating unit for a sufficientlylong time and, consequently, the gas heating unit is able to heat thedinitrogen oxide at an improved heating efficiency.

[0062] In addition, preferably, the controller is adapted to control thegas heating unit to heat the dinitrogen oxide gas to a temperature in arange of 750 to 950° C.

BRIEF DESCRIPTION OF THE DRAWINGS

[0063]FIG. 1 is a schematic view of a thermal processing system in afirst embodiment according to the present invention;

[0064]FIG. 2 is a table of concentrations of component gases ofrespective heated process gases;

[0065]FIG. 3 is a table of thicknesses and maximum nitrogen contents(peak N) of respective silicon oxynitride films;

[0066]FIG. 4 is a table of temperatures of an inner tube when a heatedprocess gas is supplied into the inner tube;

[0067]FIG. 5 is a table of increments in the thickness and maximumnitrogen contents of respective silicon oxynitride films;

[0068]FIG. 6 is a longitudinal sectional view of a silicon dioxide filmforming system in a second embodiment according to the present inventionfor carrying out a silicon dioxide film forming method according to thepresent invention;

[0069]FIG. 7 is a schematic perspective view of an essential part of thesilicon dioxide film forming system of FIG. 6;

[0070]FIG. 8 is a sectional view of a gas heating unit included in thesilicon dioxide film forming system of FIG. 6;

[0071]FIG. 9 is a graph showing results of film thickness measurementsperformed to examine the dependence of film thickness uniformity onwafer boat portions;

[0072]FIG. 10 is a graph showing results of experiments conducted toexamine the relation between oxidation time and film thicknessuniformity;

[0073]FIG. 11 is a table of measured hydrogen concentrations of anatmosphere in the reaction tube at a region near an exhaust port, inconditions where a process gas is heated by a gas heating unit and inconditions where the process gas is not heated;

[0074]FIG. 12 is a schematic view of a thermal processing system in athird embodiment according to the present invention;

[0075]FIG. 13 is a typical view of a part of the film forming system ofFIG. 12 in the vicinity of a gas heating unit;

[0076]FIG. 14 is a table showing deposition rates at which siliconnitride films were deposited and refractive indices of the siliconnitride films;

[0077]FIG. 15 is a schematic view of a thermal processing system in afourth embodiment according to the present invention;

[0078]FIG. 16 is a typical view of a part of the film forming system ofFIG. 15 in the vicinity of a gas heating unit;

[0079]FIG. 17 is a table showing the relation between temperatures ofthe gas heating unit and oxygen amounts;

[0080]FIG. 18 is a table showing the relation between temperatures ofthe gas heating unit and film deposition rates; and

[0081]FIGS. 19A to 19C are schematic views explaining problems in aconventional silicon dioxide film forming method.

BEST MODE FOR CARRYING OUT THE INVENTION

[0082] A batch-type vertical thermal processing system in a firstembodiment according to the present invention will be described asapplied to forming an oxynitride film by an oxynitride film formingmethod according to the present invention.

[0083] Referring to FIG. 1, a thermal processing system 1 has asubstantially cylindrical reaction tube 2 set in a vertical posture. Thereaction tube 2 is a double-wall structure having an inner tube 3 and anouter tube 4 having a closed upper end. The outer tube 4 surrounds theinner tube 3 so as to form an annular space of a predetermined thicknessbetween the inner tube 3 and the outer tube 4. The inner tube 3 and theouter tube 4 are formed of a heat-resisting material, such as quartz(crystal).

[0084] A cylindrical manifold 5 made of a stainless steel (SUS) isdisposed under the outer tube 4. A lower end of the outer tube 4 isjoined hermetically to the manifold 5. The inner tube 3 is supported ona support ring 6, which is formed integrally with the manifold 5 andprojecting from the inner circumference of the manifold 5.

[0085] A lid 7 is disposed below the manifold 5. A boat elevator 8 isadapted to move the lid 7 vertically. When the lid 7 is raised by theboat elevator 8, an open lower end of the manifold 5 is closed.

[0086] A wafer boat 9 made of, for example, quartz is mounted on the lid7. The wafer boat 9 can hold a plurality of objects to be processed,such as semiconductor wafers 10, at predetermined vertical intervals.

[0087] A heat insulating member 11 surrounds the reaction tube 2. Areaction tube heater 12, such as a resistance-heating element, isprovided on an inner circumference of the heat insulating member 11.

[0088] A gas supply pipe 13 is connected to a side wall of the manifold5. The gas supply pipe 13 is connected to a part of the side wall of themanifold below the support ring 6 so as to open into a space defined bythe inner tube 3. Thus, a process gas is adapted to be supplied throughthe gas supply pipe 13 into the inner tube 3 of the reaction tube 2.

[0089] A discharge port 14 is formed in a part of the side wall of themanifold 5 on a level above that of the support ring 6. The dischargeport 14 opens into the annular space between the inner tube 3 and theouter tube 4 of the reaction tube 2. A process gas is supplied throughthe gas supply pipe 13 into the inner tube 3 and a film forming processis started. Reaction products produced by the film forming process flowthrough the annular space between the inner tube 3 and the outer tube 4and are discharged from the thermal processing system 1 through thedischarge port 14.

[0090] A gas heating unit 15 provided with, for example, a resistanceheating element is combined with the gas supply pipe 13. The heatingunit 15 is adapted to heat the process gas being dinitrogen oxide (N₂ 0)gas that flows through the heating unit 15, to a predeterminedtemperature. The heated process gas flows through the gas supply pipe 13into the reaction tube 2.

[0091] A controller 16 is connected to the boat elevator 8, the reactiontube heater 12, the gas supply pipe 13 and the heating unit 15. Thecontroller 16 comprises a microprocessor, a process controller or thelike. The controller 16 is adapted to measure temperatures and pressuresof predetermined parts of the thermal processing system 1, and providecontrol signals or the like to the aforesaid components on the basis ofmeasured data, in order to control them.

[0092] An oxynitride film forming method that uses the thermalprocessing system 1 will be described as applied to forming siliconoxynitride films on semiconductor wafers 10. With respect to thefollowing description, the controller 16 controls operations of theaforesaid components of the thermal processing system 1.

[0093] The boat elevator 8 lowers the lid 7, and the wafer boat 9holding the semiconductor wafers 10 is placed on the lid 7. Then, theboat elevator 8 raises the lid 7 to load the wafer boat 9 holding thesemiconductor wafers 10 into the reaction tube 2. Thus, thesemiconductor wafers 10 are held (contained) inside the inner tube 3 ofthe reaction tube 2 and the reaction tube 2 is sealed.

[0094] The heating unit 15 is heated to a predetermined temperature by aheater, not shown. The process gas was supplied through the heating unit15 while the heating unit 15 was heated at 750° C., 900° C. or 1000° C.,so that the concentrations of the components of the process gas heatedby the heating unit 15 were measured to examine the effect of thetemperature of the heating unit 15. The measured concentrations (molepercentages) of the components of the process gas in the respectiveconditions are shown in FIG. 2.

[0095] As obvious from FIG. 2, about half the dinitrogen oxide was notpyrolyzed at 750° C. At 900° C., 8% of the dinitrogen oxide was notpyrolyzed. At 1000° C., 1% of the dinitrogen oxide was not pyrolyzed.Thus, it was confirmed that dinitrogen oxide can be substantiallycompletely pyrolyzed when the heating unit 15 is heated at a temperaturenot lower than 900° C. It was also found that nitrogen, oxygen, nitrogenmonoxide and nitrogen dioxide or the like are produced by the pyrolysisof dinitrogen oxide.

[0096] The nitrogen concentration increased from 28% to 40% when thetemperature was raised from 750° C. to 900° C. The nitrogenconcentration greatly increased to 47% when the temperature was raisedto 1000° C. That is, it was found that a large amount of nitrogen can besupplied to the semiconductor wafers 10 when the temperature is notlower than 900° C. On the other hand, oxygen concentration, as comparedwith nitrogen concentration, did not increase significantly when thetemperature was raised from 750° C. through 900° C. to 1000° C., becausethe ratio of decrease of oxygen is greater than that of nitrogen sincenitrogen monoxide and nitrogen dioxide are produced from nitrogen andoxygen. Thus, it was confirmed that the nitrogen concentration of theprocess gas increases and the increase of the oxygen concentration isnot as great as that of nitrogen concentration when dinitrogen oxide isheated to temperatures not lower than 900° C. That is, because theincrease of the nitrogen concentration of the process gas is largerelative to that of the oxygen concentration of the same when theprocess gas is heated to temperatures not lower than 900° C., a largeamount of nitrogen can be supplied to the semiconductor wafers 10.

[0097] Preferably, the heating unit 15 is heated to a temperature of900° C. or above, at which dinitrogen oxide, i.e., the process gas, issubstantially completely pyrolyzed. Since only 1% of the dinitrogenoxide is not pyrolyzed when the dinitrogen oxide is heated at 1000° C.,further pyrolysis of the dinitrogen oxide cannot be expected even if thedinitrogen oxide is heated to a higher temperature of, for example,1100° C. Thus, it is most preferable to heat the heating unit 15 toabout 1000° C. In this embodiment, the heating unit 15 is heated at1000° C.

[0098] The reaction tube heater 12 heats the interior of the reactiontube 2 to a predetermined temperature, such as 800° C., lower than thetemperature to which the process gas is heated. The temperature of thereaction tube 2 is determined according to thickness of a siliconoxynitride film to be formed, and is a temperature lower than thetemperature to which the process gas is heated and high enough to form asilicon oxynitride film. Preferably, the temperature of the reactiontube 2 is, for example, in a range of 750 to 850° C. The thickness ofthe silicon oxynitride film is dependent on the temperature of thereaction tube 2 and the duration of supply of the process gas. A siliconoxynitride film having a predetermined nitrogen content and a desiredthickness cannot be formed if the temperature of the reaction tube 2 isbelow 750° C. On the other hand, an oxide film grows greatly and thenitrogen content of the silicon oxynitride film decreases if thetemperature of the reaction tube 2 is above 850° C. In addition, if thereaction tube 2 is heated to a temperature not higher than 750° C. andthe process gas is supplied for a long time, in some cases, the amountof nitrogen diffused in the film may be saturated. Therefore, it is morepreferable that the temperature of the reaction tube 2 is in a range of800 to 850° C.

[0099] After the reaction tube 2 has been sealed, the reaction tube 2 isevacuated to a predetermined pressure of, for example, 95760 Pa (720Torr). Then, dinitrogen oxide gas is supplied, for example, at 5 l/min(5 slm) into the gas supply pipe 13, maintaining the pressure in thereaction tube 2 at 95760 Pa (720 Torr).

[0100] The heating unit 15 pyrolyzes the dinitrogen oxide gas (processgas) introduced into the gas supply pipe 13. The pyrolyzed process gasis supplied through the gas supply pipe 13 onto the semiconductor wafers10 placed inside the inner tube 3.

[0101] In the reaction tube 2, surfaces of the semiconductor wafers 10are oxynitrided by the pyrolyzed process gas. The process gas issupplied for a predetermined time of, for example, 15 min, siliconoxynitride films are formed on the semiconductor wafers 10,respectively. FIG. 3 shows thicknesses and maximum nitrogen contents(Peak N) of silicon oxynitride films formed on semiconductor wafers. Themaximum nitrogen content (Peak N) is the greatest one of respectivenitrogen contents of different parts of a silicon oxynitride film, andis a value that serves as a criterion on which the estimation of thenitrogen content of the silicon oxynitride film is based. Comparativesilicon oxynitride films of qualities as shown in FIG. 3 were formed bysilicon oxynitride film forming methods of Comparative examples 1 and 2,wherein the process gas was not heated by the heating unit 15 and thereaction tube 2 was heated at 800° C. and 900° C., respectively.

[0102] As shown in FIG. 3, the silicon oxynitride film formed by thesilicon oxynitride film forming method of the present invention(Example) had a maximum nitrogen content of 2.24 atomic percent even ifthe temperature of the reaction tube 2 was 800° C., which is lower thanconventional temperature. That is, the maximum nitrogen content of 2.24atomic percent of the silicon oxynitride film formed by the siliconoxynitride film forming method in the Example was comparable to themaximum nitrogen content of 2.33 atomic percent of the siliconoxynitride film formed by the silicon oxynitride film forming method inthe Comparative example 1 wherein the reaction tube 2 is heated to 900°C., which may be due to the increase of the nitrogen concentration ofthe process gas resulting from the pyrolysis of dinitrogen oxide by theheating unit 15.

[0103] The reduction of the temperature of the reaction tube 2 to 800°C. caused the reduction of oxidation rate and, consequently, the siliconoxynitride film as thin as 2 nm could be formed. That is, the siliconoxynitride film forming method of the present invention was able to forma thin silicon oxynitride film without reducing maximum nitrogencontent, whereas the silicon oxynitride film formed by the siliconoxynitride film forming method in the Comparative example 2, wherein thereaction tube 2 is heated to 800° C., was thin but had a low maximumnitrogen content. In addition, since the heating unit 15 pyrolyzed thedinitrogen oxide substantially completely, the silicon oxynitride filmhad an excellent intrasurface thickness uniformity.

[0104] The supply of the process gas through the gas supply pipe 13 isstopped after desired silicon oxynitride films have been formed on thesurfaces of the semiconductor wafers 10. The gas prevailing in thereaction tube 2 is discharged through the discharge port 14 and thepressure in the reaction tube 2 returns to the atmospheric pressure.Then, the boat elevator 8 lowers the lid 7 to unload the wafer boat 9holding the semiconductor wafers 10 from the reaction tube 2.

[0105] The temperature of the inner tube 3 was measured to examine theeffect of supplying the process gas heated at the temperature (1000° C.)higher than the temperature (800° C.) of the reaction tube 2 into thereaction tube 2 on the temperature of the inner tube 3. Temperature wasmeasured at four measuring points T1 to T4 on the inner circumference ofthe inner tube 3, as shown in FIG. 1. Measured temperatures are shown inFIG. 4. Measured temperatures of the inner tube 3 in Comparative example3 wherein the process gas is not heated are also shown in FIG. 4 forcomparison. As obvious from FIG. 4, whereas the temperature of theprocess gas used in the silicon oxynitride film forming method of thepresent invention and that of the same used in the silicon oxynitridefilm forming method in Comparative example 3 were different, thetemperatures of parts of the inner circumference of the inner tube 3during the execution of the silicon oxynitride film forming method ofthe present invention were substantially equal to those of the sameparts of the inner tube 3 during the execution of the silicon oxynitridefilm forming method in Comparative example 3. Thus, it was confirmedthat the supply of the heated process gas into the reaction tube 2 doesnot disturb the uniformity of temperature distribution in the reactiontube 2.

[0106] As apparent from the foregoing description, according to thepresent embodiment, the process gas is heated to 1000° C. by the heatingunit 15, and the substantially completely pyrolyzed process gas issupplied into the reaction tube 2. Consequently, the nitrogenconcentration of the process gas can be increased and a large amount ofnitrogen can be supplied onto the semiconductor wafers 10. Therefore,even if the temperature of the reaction tube 2 is reduced from 900° C.to 800° C., a silicon oxynitride film having a maximum nitrogen contentsubstantially equal to that of a silicon oxynitride film formed by thesilicon oxynitride film forming method that heats the reaction tube at900° C. can be formed. In addition, since the temperature of thereaction tube 2 is reduced from 900° C. to 800° C., a very thin siliconoxynitride film can be formed.

[0107] Modifications of the silicon oxynitride film forming method inthe first embodiment are possible.

[0108] The foregoing silicon oxynitride forming method in the firstembodiment subjects the semiconductor wafers 10 directly to theoxynitriding process to form the silicon oxynitride film on thesemiconductor wafers 10. However, semiconductor wafers having surfacescoated respectively with silicon dioxide films may be subjected to theoxynitriding process to form silicon oxynitride films on thesemiconductor wafers.

[0109] Semiconductor wafers 10 having surfaces coated respectively with,for example, 3 nm thick silicon dioxide films were held on the waferboat 9. The pressure in the reaction tube 2 was set at 95760 Pa (720Torr), the heating unit 15 was heated to 900° C. or 1000° C. Thereaction tube 2 was heated to 750° C., 800° C. or 850° C. Dinitrogenoxide gas was supplied through the gas supply pipe 13 at 5 l/min (5 slm)for 15 min to form silicon oxynitride films on the semiconductor wafers10, respectively, by subjecting the silicon dioxide films to anoxynitriding process. FIG. 5 shows thickness increment and maximumnitrogen content of the thus formed silicon oxynitride films. FIG. 5also shows those of cases wherein the temperature of the heating unit 15is 1000° C. and the process gas is supplied for 30 min, and caseswherein the heating unit 15 is not heated.

[0110] As obvious from FIG. 5, heating the process gas by the heatingunit 15 can increase maximum nitrogen content. The thicknesses aresubstantially the same when the processes are the same in thetemperature of the reaction tube 2 and reaction time. Thus, a very thinsilicon oxynitride film having a desired nitrogen content can be formedby heating the process gas by the heating unit 15 and by heating thereaction tube 2 to a reduced temperature.

[0111] For example, a very thin silicon oxynitride film having a maximumnitrogen content equal to that of a silicon oxynitride film (thicknessincrement: 1.01 nm, maximum nitrogen content (Peak N): 0.52 atomicpercent) formed by heating the reaction tube 2 to 850° C. and withoutheating the process gas by the heating unit 15 can be formed by heatingthe process gas to 900° C. by the heating unit 15 and by heating thereaction tube 2 to 750° C. The silicon oxynitride film thus formed canhave a thickness increment of 0.29 nm, which is about ¼ of the thicknessincrement of 1.01 nm.

[0112] When the respective temperatures of the heating unit 15 and thereaction tube 2 were 1000° C. and 750° C. and the reaction time wasincreased from 15 min to 30 min, the maximum nitrogen content (Peak N)decreased from 1.13 atomic percent to 0.78 atomic percent. It may beconsidered that, in some cases, the amount of nitrogen diffused in thefilm is saturated and, consequently, the thickness increases, when thetemperature of the reaction tube 2 is comparatively low and the reactiontime is long. Thus, it is preferable to heat the reaction tube 2 to atemperature not lower than 800° C. when the reaction time is as long as30 min.

[0113] The oxynitride film forming system in the first embodiment is abatch-type vertical thermal processing system provided with the reactiontube 2 of a double-wall structure consisting of the inner tube 3 and theouter tube 4. The present invention is not limited thereto, and isapplicable to various processing systems for forming an oxynitride filmon an object to be processed. The object to be processed is not limitedto a semiconductor wafer and the present invention is applicable toprocessing various objects, such as glass substrates for forming LCDs.

[0114]FIG. 6 shows a silicon dioxide film forming system in a secondembodiment according to the present invention for carrying out a silicondioxide film forming method according to the present invention. Thesilicon dioxide film forming system includes a vertical thermalprocessing unit 101 and a heating unit 102 that can heat a process gasto be supplied to the vertical thermal processing unit 101. As shown inFIGS. 6 and 7, the vertical thermal processing unit 101 comprises avertical thermal processing furnace 103, a wafer boat 104 as a waferholder, a boat elevator 140 for vertically moving the wafer boat 104,and a gas supply pipe 105 and an exhaust pipe 130 connected to thethermal processing furnace 103.

[0115] The vertical thermal processing furnace 103 includes a reactiontube 131, i.e., a reaction vessel, made of, for example, quarts, areaction tube heater 132 provided with a resistance heating element andsurrounding the reaction tube 131, and a liner soaking tube 133interposed between the reaction tube 131 and the reaction tube heater132 and supported on a heat insulating member 134. The reaction tube 131has an open lower end and a top wall 131 a. A gas diffusing plate 131 cprovided with a plurality of holes 131 b is disposed in the reactiontube 131 at a position at a short distance below the top wall 131 a. Thegas supply pipe 105 extends through the heat insulating member 134,bends at right angle at a position on the inner side of the heatinsulating member 134, extends upright through a space between thereaction tube 131 and the liner tube 133 and extends into a spacebetween the top wall 131 a of the reaction tube 131 and the gasdiffusing plate 131 c.

[0116] As shown in FIG. 7, the wafer boat 104 has a top plate 141, abottom plate 142 and a plurality of support bars 143 extended betweenthe top plate 141 and the bottom plate 142. Each of the support bars 143is provided with horizontal grooves to hold wafers W therein. The waferboat 104 is mounted on a heat insulating cylinder 145 placed on a lid144 for closing the open lower end 135 (FIG. 6) of the reaction tube131. The heat insulating cylinder 145 is supported on a turntable 146(FIG. 6) connected to a shaft 147. The shaft 147 is adapted to be drivenfor rotation by a driving unit M disposed on the boat elevator 140 (FIG.6), to rotate the turntable 146 supporting the wafer boat 104. The lid144 is moved vertically by the boat elevator 140 to carry the wafer boat104 into and out of the thermal processing furnace 103.

[0117] Referring to FIG. 6, the heating unit 102 is placed in a part ofthe gas supply pipe 105 extending outside the vertical thermalprocessing unit 101. As shown in FIG. 8, the heating unit 102 comprisesa heating pipe 121 made of, for example, transparent quartz defining aheating chamber, a heating element 122 helically wound around theheating pipe 121, and a cylindrical heat insulating member 123 coveringthe heating pipe 121 and the heating element 122. A cooling waterpassage 124 is formed in the heat insulating member 123. A coolingmedium, such as cooling water, is passed through the cooling waterpassage 124. For example, many transparent quartz beads 120 as flowimpeding members are packed in the heating pipe 121 to extend thestaying time of a gas in the heating pipe 121. The quartz beads 120exert resistance against the flow of the gas in the heating pipe 121.The quartz beads 120 are heated and the gas flows touching the heatedquartz beads 120, whereby the gas is heated efficiently.

[0118] The heating element 122 is a carbon braid formed by braiding aplurality of carbon fiber strands of a high purity scarcely containingmetallic impurities. Electric power is supplied through a cable 125 tothe heating element 122 to generate heat. Preferably, the heating unit102 is provided with a temperature sensor 126, such as a thermocouple.

[0119] As shown in FIG. 6, a valve V0 is placed in a part of the gassupply pipe 105 on an upstream side of the heating unit 102, and branchpipes 151 and 152 are connected to the valve V0. The branch pipes 151and 152 are connected to an oxygen gas source 153 and a hydrogenchloride gas source 154, respectively. Shown also in FIG. 6 are valvesV1 and V2, and mass flow controllers MF1 and MF2, i.e., flow controllersfor controlling the flow rate of the gas. Preferably, the heating unit102 is disposed as close to the thermal processing furnace 103 aspossible to prevent the heated gas from cooling before flowing into thethermal processing furnace 103.

[0120] The operation of the silicon dioxide film forming system in thesecond embodiment will be described.

[0121] A plurality of wafers W, such as sixty wafers W, each having asurface provided with a silicon layer, are held on the wafer boat 104 ina tier-like manner. Then, the boat elevator 140 carries the wafer boat104 into the reaction tube 131 that has been heated beforehand at apredetermined temperature by the reaction tube heater 132, and the openlower end 135 of the reaction tube is closed hermetically by the lid 144as shown in FIG. 6. Subsequently, the interior of the reaction tube 131is heated to a predetermined process temperature of, for example, 800°C. In the step of loading the wafers W into the reaction tube 131 andthe step of heating the interior of the reaction tube 131, nitrogen gascontaining a small amount of oxygen gas is supplied through a nitrogengas supply pipe, not shown, into the reaction tube 131. After theinterior of the reaction tube 131 has been heated at the processtemperature, the supply of nitrogen gas is stopped and the gas remainingin the reaction tube 131 is discharged through the exhaust pipe 130 byan evacuating device, not shown, to evacuate the reaction tube 131 to alow negative pressure. An oxidation process is started after thetemperature of the wafers W has stabilized.

[0122] The heating unit 102 disposed outside the vertical thermalprocessing unit 101 is energized to heat the interior of the heatingpipe 121 to, for example, 1000° C. The valve V₀ is opened to pass aprocess gas containing oxygen gas and hydrogen chloride gas through theheating pipe 121. The process gas is heated at about 1000° C. as thesame flows through gaps between the transparent quartz beads 120touching the transparent quartz beads 120. It is considered that theoxygen gas and the hydrogen chloride gas of the process gas may undergochemical reactions expressed by the following reaction formulas, andthat a very small amount of steam on the order of several hundreds partsper million may be generated.

2HCl→H₂+Cl₂

H₂+½O₂→H₂O

[0123] The thus heated process gas flows through a part of the gassupply pipe 105 connected to the thermal processing furnace 103 and apart of the same extended along the inner surface of the liner tube 133and flows into an upper part of the reaction tube 131. Then, the processgas flows through the holes 131 b into a processing region in thereaction tube 131, and is discharged through the exhaust pipe 130connected to a lower part of the reaction tube 131. The process gasflows into spaces between the stacked wafers W. The oxygen gas containedin the process gas oxidizes the silicon layers on the surfaces of thewafers W to form silicon dioxide films. Steam contained in the processgas of a small concentration promotes the formation of the silicondioxide films.

[0124] The silicon dioxide film forming system in the second embodimentis capable of forming silicon dioxide films, each having a highintrasurface thickness uniformity and a high interwafer thicknessuniformity. This may be considered to be due to the following reasons.

[0125] The process gas, i.e., a mixture of oxygen gas and hydrogenchloride gas, is heated by the heating unit 102 to, for example, about1000° C., so that steam is generated. The temperature of the process gasslightly drops while the process gas is flowing through the gas supplypipe 105 after being heated. However, the amount of thus generated steamdoes not decrease even if the temperature of the process gas drops; thatis, equilibrium of the chemical reaction of oxygen and hydrogen toproduce steam does not shift toward the side of the product. Therefore,process gas does not produce steam any further in the reaction tube 131after steam has been produced at a temperature higher than the processtemperature in the reaction tube 131.

[0126] Thus, the generation of steam has been substantially terminatedbefore the process gas flows into spaces between the wafers W held onthe wafer boat 104. Therefore, the amount of steam contained in theprocess gas flowing from the periphery toward the center of each wafer Wremains substantially constant regardless of position thereof, and hencethe film formation promoting effect of steam on all parts of thesurfaces of the wafers W held in an upper part of the wafer boat 104 issubstantially the same. Consequently, a film having a high intrasurfacethickness uniformity can be formed.

[0127] When the process gas is supplied by the conventional process gassupplying method, the generation of steam increases progressively towardthe lower part of the wafer boat 104, and hence films of unsatisfactoryintrasurface thickness uniformity are formed on the wafers W held in theupper part of the wafer boat 104 while films of higher intrasurfacethickness uniformity are formed on the wafers W held in the lower partof the wafer boat 104. When the process gas is supplied by a process gassupplying method according to the present invention, an atmosphere thathas been created by the conventional process gas supplying method in alower part of the wafer boat 104 can be created in both upper and lowerparts of the wafer boat 104. Consequently, films can be formed on thewafers W held on the wafer boat 104 in high interwafer thicknessuniformity.

[0128] More strictly, it is considered that the steam concentration ofthe process gas decreases as the process gas flows toward the center ofthe wafer W because the steam contributes to the promotion of filmformation. However, as mentioned above, the temperature of a centralpart of the wafer W is higher than that of a peripheral part of thesame, and hence there is a tendency for the thickness of the film formedon the wafer W to increase from the periphery toward the center of thewafer W. The relatively high film formation promoting effect of steamaround the periphery of the wafer W contributes to increasing thethickness of a part of the film on the peripheral part of the wafer Wand, consequently, the intrasurface thickness uniformity of the film maybe further improved.

[0129] The effect of progressive steam generation in the reaction tube103 on the intrasurface thickness uniformity and the interwaferthickness uniformity is greater under lower temperatures. Thus, thesecond embodiment can greatly contribute to the reduction of the processtemperature.

[0130] A compound gas containing hydrogen and chlorine other than thehydrogen chloride gas may be used. For example, dichlorosilane gas(SiH₂Cl₂ gas) may be used instead of the hydrogen chloride gas. The stepof producing water by supplying energy to the process gas is not limitedto the step of heating the process bas by the heating unit 102. Watermay be produced by a step wherein the process gas is activated bysupplying energy to the process gas with, for example, power ofmicrowaves or a laser beam. In that case too, preferably, steam isgenerated in advance before the process gas is supplied into thereaction tube such that steam is not generated any further after theprocess gas has been supplied into the reaction tube.

[0131] The oxidation process for oxidizing wafers in the reaction vesselmay be carried out by a single-wafer thermal processing system, insteadof a batch thermal processing system.

[0132] Results of experimental film forming operations with theforegoing silicon dioxide film forming system will be describedhereinafter.

[0133] (Experiment 1)

[0134] Silicon dioxide films were formed on the surfaces of 20 cmdiameter wafers under the following process conditions, respectively.

[0135] Temperature in reaction tube: 800° C.

[0136] Flow rate of gases: O₂/HCl=10/0.5 slm

[0137] Processing time: 90 min

[0138] Temperature of heating unit: 1000° C.

[0139] Number of wafers on wafer boat: 100

[0140] Pressure in reaction tube: −49 Pa (−5 mmH₂O)

[0141] The thickness of silicon oxide films formed on the wafers inupper, middle and lower parts, respectively, of the wafer boat wasmeasured to examine the intrasurface thickness uniformity of the silicondioxide films. In addition, silicon dioxide films were formed on thesurfaces of wafers under process conditions similar to the foregoingprocess conditions, except that the heating element of the heating unitwas not energized. FIG. 9 shows the measured results. Intrasurfacethickness uniformity is represented by a value calculated by using:

[{(Maximum thickness)−(Minimum thickness)}/2×(Mean thickness)]×100 (%)

[0142] As obvious form FIG. 9, supplying the process gas after heatingthe same into the reaction tube improves the intrasurface thicknessuniformity of the silicon dioxide films formed on the wafers in theupper and the middle parts of the wafer boat and improves the interwaferthickness uniformity as well.

[0143] (Experiment 2)

[0144] Silicon dioxide films were formed on the surfaces of 20 cmdiameter wafers under the following process conditions, respectively.

[0145] Temperature in reaction tube: 800° C.

[0146] Flow rate of gases: O₂/HCl=10/0.3 slm

[0147] Temperature of heating unit: 1000° C.

[0148] Number of wafers on wafer boat: 100

[0149] Pressure in reaction tube: −49 Pa (−5 mmH₂O)

[0150] Processing time: 2, 15, 30, and 60 min

[0151] Intrasurface thickness uniformity of the wafers held in a middlepart of the wafer boat was examined. Interwafer thickness uniformity wasalso examined. FIG. 10 shows the results of examinations. Interwaferthickness uniformity is represented by a value calculated by using:

(A/2×B)×100 (%)

[0152] where A is the difference between the maximum and the minimumamong the respective mean thicknesses of the silicon dioxide filmsformed on the wafers held on the wafer boat (practically, apredetermined number of monitor wafers held on the wafer boat) and B isthe mean of the respective mean thicknesses of the silicon dioxide filmsformed on the wafers.

[0153] As obvious from FIG. 10, the longer the processing time, i.e.,the greater the film thickness, the greater is the effect on theimprovement of the intrasurface thickness uniformity and the interwaferthickness uniformity. Especially, the intrasurface thickness uniformityand the interwafer thickness uniformity may be improved even in a thinrange of about 3 nm of the film thickness.

[0154] (Experiment 3)

[0155] An empty wafer boat was loaded into the reaction tube, theinterior of the reaction tube was heated to 800° C., and oxygen gas andhydrogen chloride gas were supplied at 10 slm and 1 slm, respectively.The hydrogen concentration of the gas discharged through the exhaustpipe was measured while the system was operating in an operating modewith the heating element energized to heat the process gas at 1000° C.and in an operating mode with the heating element not energized.

[0156]FIG. 11 shows the measured results, in which preparatory timesignifies a length of time for which the gas was supplied beforestarting analysis. It is known from FIG. 11 that the hydrogenconcentration of the discharged gas is small when the process gas isheated by the heating unit. It is inferred that the reaction: H₂+½O₂→H₂O is promoted when the process gas is heated.

[0157] A system for forming a silicon nitride film and a method offorming a silicon nitride film in a third embodiment according to thepresent invention will be described in connection with a batch-typevertical thermal processing system shown in FIG. 12.

[0158] Referring to FIG. 12, the thermal processing system 201 includesa substantially cylindrical reaction tube 202 disposed in a verticalposture. The reaction tube 202 is a double-wall structure consisting ofan inner tube 203 defining a film forming space and an outer tube 204having a closed upper end and surrounding the inner tube 203 so that anannular space of a predetermined thickness is formed between the innertube 203 and the outer tube 204. The inner tube 203 and the outer tube204 are made of a heat-resisting material, such as quartz (crystal).

[0159] A cylindrical manifold 205 made of a stainless steel (SUS) isdisposed under the outer tube 204. A lower end of the outer tube 204 isjoined hermetically to the manifold 205. The inner tube 203 is supportedon a support ring 206, which is formed integrally with the manifold 205and projecting from the inner circumference of the manifold 205.

[0160] A lid 207 is disposed below the manifold 205. A boat elevator 208is adapted to move the lid 207 vertically. When the lid 207 is raised bythe boat elevator 208, an open lower end of the manifold 205 is closed.

[0161] A wafer boat 209 made of, for example, quartz is mounted on thelid 207. The wafer boat 209 can hold a plurality of objects to beprocessed, such as semiconductor wafers 210, at predetermined verticalintervals of, for example, 10.4 mm.

[0162] A heat insulating member 211 surrounds the reaction tube 202. Areaction tube heater 212, such as a resistance-heating element, isprovided on an inner circumference of the heat insulating member 211.

[0163] A plurality of gas supply pipes are connected to a side wall ofthe manifold 205. In the third embodiment, a first gas supply pipe 213and a second gas supply pipe 214 are connected to the side wall of themanifold 205.

[0164] The first gas supply pipe 213 opens into a space defined by theinner tube 203. For example, the first gas supply pipe 213 is connectedto a part of the side wall of the manifold 205 below the support ring206, i.e., below the level of the lower end of the inner tube 203. Asilane gas, such as disilane gas (Si₂H₆ gas), is adapted to be suppliedthrough the first gas supply pipe 213 into the space defined by theinner tube 203.

[0165] The second gas supply pipe 214 opens into the space defined bythe inner tube 203. Similarly to the first gas supply pipe 213, thesecond gas supply pipe 214 is connected to a part of the side wall ofthe manifold 205 below the support ring 206, i.e., below the level ofthe lower end of the inner tube 203. Trimethylamine gas (TMA gas) as anitrogen source is adapted to be supplied through the second gas supplypipe 214 into the space defined by the inner tube 203.

[0166] A heating unit 215 provided with, for example, a resistanceheating element is combined with the second gas supply pipe 214. Theheating unit 215 is adapted to heat trimethylamine gas that flowsthrough the heating unit 215 to a predetermined temperature. The heatedtrimethylamine gas flows through the second gas supply pipe 214 into thereaction tube 202.

[0167] The second gas supply pipe 214 has a restricting part 216 in apart thereof on a downstream side of the heating unit 215. FIG. 13 is anenlarged view of a part of the second gas supply pipe 214 around therestricting part 216. As shown in FIG. 13, the restricting part 216 hasa protrusion 216 a defining an orifice 216 b. The protrusion 216 aprotrudes from the inner circumference of the second gas supply pipe 214so as to reduce the inside diameter of a section of the second gassupply pipe 214. The protrusion 216 a has the shape of a round pipe. Theinner circumference of the protrusion 216 a defines the orifice 216 b.In this embodiment, the inside diameter of the second gas supply pipe214 is 20 mm and the diameter of the orifice 216 b is about 0.6 mm.

[0168] A discharge port 217 is formed in a part of the side wall of themanifold 205 on a level above that of the support ring 206. Thedischarge port 217 opens into the annular space between the inner tube203 and the outer tube 204 of the reaction tube 202. The process gasesare adapted to be supplied through the first gas supply pipe 213 and thesecond gas supply pipe 214 into the inner tube 203 to carry out a filmforming process. Reaction products produced by the film forming processflow through the annular space between the inner tube 203 and the outertube 204 and are discharged through the discharge port 217.

[0169] An exhaust pipe 218 is connected hermetically to the dischargeport 217. The exhaust pipe 218 is provided with a valve 219 and a vacuumpump 220. The opening of the valve 219 is regulated such that theinteriors of the reaction tube 202 and the exhaust pipe 218 aremaintained at predetermined pressures, respectively. The vacuum pump 220evacuates the reaction tube 202 through the exhaust pipe 218 andoperates so as to adjust the pressures in the reaction tube 202 and theexhaust pipe 218.

[0170] A controller 221 is connected to the boat elevator 208, thereaction tube heater 212, the first gas supply pipe 213, the second gassupply pipe 214, the heating unit 215, the valve 219 and the vacuum pump220. The controller 221 may comprise a microprocessor, a processcontroller or the like. The controller 221 is adapted to measuretemperatures and pressures of predetermined parts of the thermalprocessing system 201, and provide control signals or the like to theaforesaid components on the basis of measured data to control them.

[0171] A silicon nitride film forming method that uses the thermalprocessing system 201 will be described as applied to forming siliconnitride films on semiconductor wafers 210. With respect to the followingdescription, the controller 221 controls operations of the aforesaidcomponents of the thermal processing system 201.

[0172] The boat elevator 208 lowers the lid 207, and the wafer boat 209holding the semiconductor wafers 210 is placed on the lid 207. Then, theboat elevator 208 raises the lid 207 to load the wafer boat 209 holdingthe semiconductor wafers 210 into the reaction tube 202. Thus, thesemiconductor wafers 210 are held (contained) inside the inner tube 203of the reaction tube 202 and the reaction tube 202 is sealed.

[0173] The reaction tube heater 212 heats the interior of the reactiontube 202 to a predetermined temperature. Preferably, the temperature towhich the interior of the reaction tube 202 is heated is lower than atemperature in a range of 650 to 700° C. to which the conventionalreaction tube has been heated, and suitable for forming a siliconnitride film. In detail, the temperature is preferably in a range of forexample 400 to 650° C. In the third embodiment, the interior of thereaction tube 202 is heated at 550° C.

[0174] The heating unit 215 is heated to a predetermined temperature bya heater, not shown. The temperature to which the heating unit 215 isheated is a temperature capable of preheating trimethylamine, which hasa large heat capacity and hence a difficulty to heat, so that thetrimethylamine can be pyrolyzed and can supply nitrogen when heated inthe reaction tube 202. Preferably, the heating unit 215 is heated to atemperature in a range of 500 to 700° C. The trimethylamine can not besatisfactorily heated if the temperature of the heating unit 215 isbelow 500° C., and the trimethylamine is pyrolyzed substantiallycompletely by the heating unit 215 if the heating unit 215 is heated to700° C. In the third embodiment, the heating unit 215 is heated at 550°C.

[0175] After the reaction tube 202 has been sealed, the opening of thevalve 219 is controlled and the vacuum pump 220 is operated to startevacuating the reaction tube 202. The reaction tube 202 is evacuateduntil the pressure in the reaction tube 202 is reduced from theatmospheric pressure to a predetermined pressure, such as 127 Pa (0.95Torr).

[0176] Preferably, the heating unit 215 is evacuated, for example, to apressure in a range of 20 to 90 kPa (150 to 677 Torr). In the thirdembodiment, the heating unit 215 is evacuated to 84 kPa (630 Torr).Generally, pyrolyzing efficiency (heating efficiency) is apt to decreaseunder a reduced pressure. However, since the pressure in the heatingunit 215 is higher than that in the reaction tube 202, the heating unit215 is able to heat trimethylamine at a high heating efficiency.

[0177] While the pressure in the reaction tube 202 is maintained at 127Pa (0.95 Torr), disilane is supplied through the first gas supply pipe213 into the inner tube 203 at a predetermined flow rate of, forexample, 0.025 l/min (25 sccm).

[0178] Trimethylamine is supplied through the second gas supply pipe 214to the heating unit 215 at a predetermined flow rate of, for example, 1l/min (1000 sccm). The heating unit 215 preheats the trimethylamine, andthe preheated trimethylamine is supplied through the second gas supplypipe 214 into the inner tube 203.

[0179] Since the second gas supply pipe 214 has in the part thereof onthe downstream side of the heating unit 215 the restricting part 216having the orifice 216 b, the trimethylamine stays in the heating unit215 for a sufficiently long time. Thus, the heating unit 215 is able toheat the trimethylamine at a high heating efficiency.

[0180] The disilane and the trimethylamine supplied into the inner tube203 flow over the semiconductor wafers 210 while heated and pyrolyzed.Then, the surfaces of the semiconductor wafers 210 are nitrided by thepyrolyzed process gas. That is, silicon nitride films are formed on thesemiconductor wafers 210, respectively, after the process gas has beensupplied into the reaction tube 202 for, for example, 120 min.

[0181]FIG. 14 shows deposition rate DR at which the silicon nitridefilms are deposited and refractive index RI of the silicon nitridefilms. Refractive index is an indicator of the composition (nitrogencontent) of the formed silicon nitride film. A silicon nitride filmhaving a substantially stoichiometric composition has RI=2.0. Depositionrate at which a silicon nitride film in a comparative example isdeposited and refractive index of the silicon nitride film in thecomparative example are shown also in FIG. 14 for reference. Whenforming the silicon nitride film in the comparative example, the heatingunit 215 does not preheat trimethylamine.

[0182] As obvious from FIG. 14, the silicon nitride film forming methodof the present embodiment is able to form a silicon nitride film of asubstantially stoichiometric composition having a refractive index RI of2.0, even if the temperature in the reaction tube 202 is 550° C., whichis lower than 650° C. used by a conventional method. The silicon nitridefilm forming method of the present embodiment is able to form thesilicon nitride film at a deposition rate of 0.70 nm/min, which is about2.6 times as high as a deposition rate of 0.27 nm/min at which thesilicon nitride film in the comparative example is formed.

[0183] Thus, the silicon nitride film of the substantiallystoichiometric composition can be formed even though the processtemperature of 550° C. in the reaction tube 202 is lower than 650° C.used by the conventional method. In addition, the silicon nitride filmof the substantially stoichiometric composition can be deposited at thehigh deposition rate. These are because the trimethylamine afterpreheated by the heating unit 215 tends to be easily pyrolyzed whenheated in the reaction tube 202 so that a larger amount of nitrogen isavailable.

[0184] Since the interior of the heating unit 215 is kept at 84 kPa (630Torr), the heating efficiency of the heating unit 215 is improved.Consequently, the trimethylamine can be more easily pyrolyzed by heatedin the reaction tube 202, a larger amount of nitrogen can be supplied,and the silicon nitride film of the substantially stoichiometriccomposition can be formed at a high deposition rate.

[0185] Since the second gas supply pipe 214 has in the part thereof onthe downstream side of the heating unit 215 the restricting part 216having the orifice 216 b, the trimethylamine stays in the heating unit215 for a sufficiently long time and, therefore, the heating unit 215 isable to heat the trimethylamine at a high heating efficiency.Consequently, the trimethylamine after preheated by the heating unit 215is easily subject to pyrolysis when heated in the reaction tube 202 anda larger amount of nitrogen is supplied, so that the silicon nitridefilm of the substantially stoichiometric composition can be deposited ata high deposition rate.

[0186] After the silicon nitride films have been formed on the surfacesof the semiconductor wafers 210, respectively, the supply of the processgases through the first gas supply pipe 213 and the second gas supplypipe 214 is stopped. The gases remaining in the reaction tube 202 aredischarged through the discharge port 217, and the interior of thereaction tube 202 is returned to the atmospheric pressure. Then, theboat elevator 208 unloads the wafer boat 209 holding the semiconductorwafers 210 from the reaction tube 202.

[0187] As apparent from the foregoing description, the silicon nitridefilm forming system in the third embodiment preheats the trimethylamineby means of the heating unit 215 capable of heating the trimethylamineat an improved heating efficiency, then the preheated trimethylamine issupplied into the reaction tube 202 to carry out a nitriding process.Therefore, even if the temperature in the reduction tube 202 isrelatively low, a silicon nitride film of a substantially stoichiometriccomposition can be formed at a high deposition rate.

[0188] The following changes may be made in the silicon nitride filmforming system in the third embodiment and the silicon nitride filmforming method using the same silicon nitride film forming system.

[0189] Although the pressure of 84 kPa (630 Torr) in the heating unit215 is higher than the pressure of 127 Pa (0.95 Torr) in the reactiontube 202 in the above embodiment, the pressure in the heating unit 215and that in the reaction tube 202 may be allowed to be substantiallyequal.

[0190] Although the restrictor 216 having the orifice 216 b is formed inthe part on the downstream side of the heating unit 215 of the secondgas supply pipe 214 in the above embodiment, any other flow restrictingstructure may be employed, to retard the passage of trimethylaminethrough the heating unit 215 instead of the restrictor 216. For example,the heating unit 215 may be provided with a long passage fortrimethylamine to extend time necessary for trimethylamine to passthrough the heating unit 215, which also improves the heating efficiencyof the heating unit 215.

[0191] Although the diameter of the orifice 216 b is about 0.6 mm in theabove embodiment, the diameter of the orifice 216 b is not limitedthereto, may be any diameter such that trimethylamine can be made tostay for a sufficiently long time in the heating unit 215.

[0192] A silane gas is not limited to disilane gas; monosilane gas (SiH₄gas) or dichlorosilane gas (SiH₂Cl₂ gas) may be used.

[0193] Although the thermal processing system in the above embodiment isthe batch type vertical thermal processing system having the heatingtube 202 of a double-wall structure consisting of the inner tube 203 andthe outer tube 204, the present invention is applicable to variousprocessing systems for forming a nitride film on an object to beprocessed. The object to be processed is not limited to a semiconductorwafer but may be, for example, a substrate for LCDs.

[0194] A system for forming a silicon dioxide film and a method offorming a silicon dioxide film in a fourth embodiment according to thepresent invention will be described in connection with a batch-typevertical thermal processing system shown in FIG. 15.

[0195] Referring to FIG. 15, the thermal processing system 301 includesa substantially cylindrical reaction tube 302 disposed in a verticalposture. The reaction tube 302 is a double-wall structure consisting ofan inner tube 303 defining a film forming space and an outer tube 304having a closed upper end and surrounding the inner tube 303 so that anannular space of a predetermined thickness is formed between the innertube 303 and the outer tube 304. The inner tube 303 and the outer tube304 are made of a heat-resisting material, such as quartz (crystal).

[0196] A cylindrical manifold 305 made of a stainless steel (sus) isdisposed under the outer tube 304. A lower end of the outer tube 304 isjoined hermetically to the manifold 305. The inner tube 303 is supportedon a support ring 306, which is formed integrally with the manifold 305and projecting from the inner circumference of the manifold 305.

[0197] A lid 307 is disposed below the manifold 305. A boat elevator 308is adapted to move the lid 307 vertically. When the lid 207 is raised bythe boat elevator 308, an open lower end of the manifold 305 is closed.

[0198] A wafer boat 309 made of, for example, quartz is mounted on thelid 307. The wafer boat 309 can hold a plurality of objects to beprocessed, such as semiconductor wafers 310, at predetermined verticalintervals of, for example, 5.2 mm.

[0199] A heat insulating member 311 surrounds the reaction tube 302. Areaction tube heater 312, such as a resistance-heating element, isprovided on an inner circumference of the heat insulating member 311.The reaction tube heater 312 is adapted to set the interior of thereaction tube 302 at a predetermined temperature.

[0200] A plurality of gas supply pipes are connected to a side wall ofthe manifold 305. In the fourth embodiment, a first gas supply pipe 313and a second gas supply pipe 314 are connected to the side wall of themanifold 305.

[0201] The first gas supply pipe 313 opens into a space defined by theinner tube 303. For example, the first gas supply pipe 313 is connectedto a part of the side wall of the manifold 305 below the support ring306, i.e., below the level of the lower end of the inner tube 303, asshown in FIG. 15. A silane gas, such as dichlorosilane gas (SiH₂Cl₂gas), is adapted to be supplied through the first gas supply pipe 313into the space defined by the inner tube 303.

[0202] The second gas supply pipe 314 opens into the space defined bythe inner tube 303. Similarly to the first gas supply pipe 313, thesecond gas supply pipe 314 is connected to a part of the side wall ofthe manifold 305 below the support ring 306, i.e., below the level ofthe lower end of the inner tube 303. Dinitrogen oxide gas (N₂O gas) isadapted to be supplied through the second gas supply pipe 314 into thespace defined by the inner tube 303.

[0203] A heating unit 315 provided with, for example, a resistanceheating element is combined with the second gas supply pipe 314. Theheating unit 315 is adapted to heat dinitrogen oxide gas that flowsthrough the heating unit 315 to a predetermined temperature. The heateddinitrogen oxide gas flows through the second gas supply pipe 314 intothe reaction tube 302.

[0204] The second gas supply pipe 314 has a restricting part 316 in apart thereof on a downstream side of the heating unit 315. FIG. 16 is anenlarged view of a part of the second gas supply pipe 214 around therestricting part 216. As shown in FIG. 16, the restricting part 316 hasa protrusion 316 a defining an orifice 316 b. The protrusion 316 aprotrudes from the inner circumference of the second gas supply pipe 314so as to reduce the inside diameter of a section of the second gassupply pipe 314. The protrusion 316 a has the shape of a round pipe. Theinner circumference of the protrusion 316 a defines the orifice 316 b.In this embodiment, the inside diameter of the second gas supply pipe314 is 20 mm and the diameter of the orifice 316 b is about 0.6 mm.

[0205] A discharge port 317 is formed in a part of the side wall of themanifold 305 on a level above that of the support ring 306. Thedischarge port 317 opens into the annular space between the inner tube303 and the outer tube 304 of the reaction tube 302. The process gasesare adapted to be supplied through the first gas supply pipe 313 and thesecond gas supply pipe 314 into the inner tube 303 to carry out a filmforming process. Reaction products produced by the film forming processflow through the annular space between the inner tube 303 and the outertube 304 and are discharged through the discharge port 317.

[0206] An exhaust pipe 318 is connected hermetically to the dischargeport 317. The exhaust pipe 318 is provided with a valve 319 and a vacuumpump 320. The opening of the valve 319 is regulated such that theinteriors of the reaction tube 302 and the exhaust pipe 318 aremaintained at predetermined pressures, respectively. The vacuum pump 320evacuates the reaction tube 302 through the exhaust pipe 318 andoperates so as to adjust the pressures in the reaction tube 302 and theexhaust pipe 318.

[0207] A controller 321 is connected to the boat elevator 308, thereaction tube heater 312, the first gas supply pipe 313, the second gassupply pipe 314, the heating unit 315, the valve 319 and the vacuum pump320. The controller 321 may comprise a microprocessor, a processcontroller or the like. The controller 321 is adapted to measuretemperatures and pressures of predetermined parts of the thermalprocessing system 301, and provide control signals or the like to theaforesaid components on the basis of measured data to control them.

[0208] A silicon dioxide film forming method that uses the thermalprocessing system 301 will be described as applied to forming silicondioxide films on semiconductor wafers 310. With respect to the followingdescription, the controller 321 controls operations of the aforesaidcomponents of the thermal processing system 301.

[0209] The boat elevator 308 lowers the lid 307, and the wafer boat 309holding the semiconductor wafers 310 is placed on the lid 307. Then, theboat elevator 308 raises the lid 307 to load the wafer boat 309 holdingthe semiconductor wafers 310 into the reaction tube 302. Thus, thesemiconductor wafers 310 are held (contained) inside the inner tube 303of the reaction tube 302 and the reaction tube 302 is sealed.

[0210] The reaction tube heater 312 heats the interior of the reactiontube 302 to a predetermined temperature suitable for forming a silicondioxide film in a range of, for example, 700 to 900° C.

[0211] The heating unit 315 is heated to a predetermined temperature bya heater, not shown. The relation between the temperature of the heatingunit 315 and the amount of oxygen that is produced through the pyrolysisof dinitrogen oxide was examined to find a suitable temperature to whichthe heating unit 315 is heated. FIG. 17 shows temperatures of theheating unit 315 and oxygen concentrations respectively corresponding tothe temperatures. It is known from FIG. 17 that the oxygen concentrationof the pyrolyzed gas is large when the temperature is 700° C. or above.The pyrolyzed gas having a large oxygen concentration promotes theoxidation of dichlorosilane supplied through the first gas supply pipe313. Thus, it is preferable that the heating unit 315 is heated to 700°C. or above.

[0212] The amount of oxygen produced by the pyrolysis increases sharplywhen the temperature of the heating unit 315 is 750° C. or above.Therefore, it is particularly preferable to heat the heating unit 315 to750° C. or above. However, dinitrogen oxide is pyrolyzed substantiallycompletely when heated at 950° C. Therefore, oxygen concentration doesnot increase beyond the oxygen concentration at 950° C. even ifdinitrogen oxide is heated to a temperature beyond 950° C. Thus, it isparticularly preferable that the temperature of the heating unit 315 isin a range of 750 to 950° C.

[0213] After the reaction tube 302 has been sealed, the opening of thevalve 319 is controlled and the vacuum pump 320 is operated to startevacuating the reaction tube 302. The reaction tube 302 is evacuateduntil the pressure in the reaction tube 302 is reduced from theatmospheric pressure to a predetermined pressure, such as 47 Pa (0.35Torr).

[0214] The heating unit 315 is evacuated, for example, to a pressure ina range of 0.1 to 90 kPa (0.75 to 677 Torr). In the fourth embodiment,the heating unit 315 is evacuated to 85 kPa (640 Torr). Generally,pyrolyzing efficiency (heating efficiency) is apt to decrease under areduced pressure. However, since the pressure in the heating unit 315 ishigher than that in the reaction tube 302, the heating unit 315 is ableto heat dinitrogen oxide at an improved heating efficiency.

[0215] While the pressure in the reaction tube 302 is maintained at 47Pa (0.35 Torr), dichlorosilane is supplied through the first gas supplypipe 313 into the inner tube 303 at a predetermined flow rate of, forexample, 0.15 l/min (150 sccm).

[0216] Dinitrogen oxide gas is supplied through the second gas supplypipe 314 to the heating unit 315 at a predetermined flow rate of, forexample, 0.3 l/min (300 sccm). The dinitrogen oxide gas is heated andpyrolyzed by the heating unit 315 to produce oxygen. The dinitrogenoxide gas containing the oxygen is supplied through the second gassupply pipe 314 into the inner tube 303.

[0217] Since the second gas supply pipe 314 has in the part thereof onthe downstream side of the heating unit 315 the restricting part 316having the orifice 316 b, the dinitrogen oxide stays in the heating unit315 for a sufficiently long time. Thus, the heating unit 315 is able toheat the dinitrogen oxide at a high heating efficiency and promote thepyrolysis of the dinitrogen oxide.

[0218] The oxygen supplied into the inner tube 303 oxidizes thedichlorosilane to produce silicon dioxide (SiO₂). Since the dinitrogenoxide is supplied into the inner tube 303 after heated at 700° C. orabove, further heating the dinitrogen oxide in the inner tube 303promotes the pyrolysis of the dinitrogen oxide. Consequently, the oxygenconcentration of the gas in the inner tube 303 increases, the oxidationof the dichlorosilane supplied into the inner tube 303 can be promoted,and the production of silicon dioxide can be increased.

[0219] The produced silicon dioxide deposits on the semiconductor wafers310. Silicon dioxide films are formed on the semiconductor wafers 310,respectively, after dichlorosilane and dinitrogen oxide have beensupplied into the reaction tube 302 for a predetermined time of, forexample, 60 min. Since the production of silicon dioxide is promoted,the silicon dioxide films can be deposited on the semiconductor wafers310 at an increased deposition rate.

[0220]FIG. 18 shows deposition rates DR at which the silicon dioxidefilms were deposited when the heating unit 315 was heated at differenttemperatures. Deposition rate at which a silicon dioxide film was formedwhen dinitrogen oxide was not heated by the heating unit 315 is shown inFIG. 18 as a comparative example.

[0221] As obvious from FIG. 18, heating dinitrogen oxide to 700° C. orabove by the heating unit 315 increases silicon dioxide film depositionrate; that is, the silicon dioxide film deposition rate increasesaccording to the increase of the amount of oxygen produced by thepyrolysis of the dinitrogen oxide. Oxygen produced through the pyrolysisof the dinitrogen oxide promotes the oxidation of dichlorosilane, andthe deposition rate of the silicon dioxide films formed on thesemiconductor wafers 310 is increased.

[0222] Since dinitrogen oxide not pyrolyzed when heated by the heatingunit 315 is at least preheated, the same is easily subject to pyrolysiswhen heated in the inner tube 303. Consequently, the dinitrogen oxidecan be efficiently pyrolyzed. Thus, the dichlorosilane can be oxidizedefficiently, and the silicon dioxide films can be formed on thesemiconductor wafers 310 at an increased deposition rate.

[0223] The facts that heating dinitrogen oxide to 750° C. or above bythe heating unit 315 increases silicon dioxide film deposition rategreatly and that silicon dioxide film deposition rate does not increasebeyond a level achieved when dinitrogen oxide is heated at 950° C. evenif dinitrogen oxide is heated to a temperature above 950° C. correspondto the variation of oxygen concentration with respect to the temperatureof the heating unit 315 shown in FIG. 17. Thus, it is particularlypreferable to heat dinitrogen oxide by the heating unit 315 to atemperature in a range of 750 to 950° C.

[0224] Since the interior of the heating unit 315 is kept at 84 kPa (630Torr), the heating efficiency of the heating unit 315 is improved.Consequently, the pyrolysis of the dinitrogen oxide is promoted, and thesilicon dioxide films can be formed at a high deposition rate.

[0225] Since the second gas supply pipe 314 has in the part thereof onthe downstream side of the heating unit 315 the restricting part 316having the orifice 316 b, the dinitrogen oxide stays in the heating unit315 for a sufficiently long time. Thus, the heating unit 315 is able toheat the dinitrogen oxide at a high heating efficiency. Consequently,the pyrolysis of the dinitrogen oxide is promoted, and the silicondioxide film deposition rate can be enhanced.

[0226] After the silicon dioxide films have been formed on the surfacesof the semiconductor wafers 310, respectively, the supply of the processgases through the first gas supply pipe 313 and the second gas supplypipe 314 is stopped. The gases remaining in the reaction tube 302 aredischarged through the discharge port 317, and the interior of thereaction tube 302 is returned to the atmospheric pressure. Then, theboat elevator 308 unloads the wafer boat 309 holding the semiconductorwafers 310 from the reaction tube 302.

[0227] As apparent from the foregoing description, the silicon dioxidefilm forming system in the fourth embodiment supplies dinitrogen oxideinto the inner tube 303 after heating the same by means of the heatingunit 315 to 700° C. or above. Therefore, the pyrolysis of the dinitrogenoxide is promoted, and the silicon dioxide film deposition rate can beenhanced.

[0228] The following changes may be made in the silicon dioxide filmforming system in the fourth embodiment and the silicon dioxide filmforming method using the same silicon dioxide film forming system.

[0229] A silane gas is not limited to dichlorosilane gas; monosilane gas(SiH₄ gas) or disilane gas (Si₂H₆ gas) may be used.

[0230] Although the pressure of 85 kPa (640 Torr) in the heating unit315 is higher than the pressure of 47 Pa (0.35 Torr) in the reactiontube 302 in the above embodiment, the pressure in the heating unit 315and that in the reaction tube 302 may be allowed to be substantiallyequal.

[0231] Although the diameter of the orifice 316 b is about 0.6 mm in theabove embodiment, the diameter of the orifice 316 b is not limitedthereto, may be any diameter such that dinitrogen oxide can be made tostay for a sufficiently long time in the heating unit 315. Although therestrictor 316 having the orifice 316 b is formed in the part on thedownstream side of the heating unit 315 of the second gas supply pipe314 in the above embodiment, any other flow restricting structure may beemployed to retard the passage of dinitrogen oxide through the heatingunit 315 instead of the restrictor 316. For example, the heating unit315 may be provided with a long passage for dinitrogen oxide to extendtime necessary for dinitrogen oxide to pass through the heating unit315, which also improves the heating efficiency of the heating unit 315.

[0232] Although the thermal processing system in the above embodiment isthe batch type vertical thermal processing system having the heatingtube 302 of a double-wall structure consisting of the inner tube 303 andthe outer tube 304, the present invention is applicable to variousprocessing systems for forming an oxide film on an object to beprocessed. The object to be processed is not limited to a semiconductorwafer but may be, for example, a substrate for LCDs.

[0233] As apparent from the foregoing description, one feature of thepresent invention enables to form a thin oxynitride film having adesired nitrogen content.

[0234] One feature of the present invention enables to form an oxidefilm having a high thickness uniformity by subjecting an object to beprocessed to a dry oxidation process, and enables the reduction ofprocess temperature.

[0235] One feature of the present invention enables to form a siliconnitride film of a substantially stoichiometric composition at a lowtemperature at a high deposition rate.

[0236] One feature of the present invention enables to form a silicondioxide film on an object to be processed at a high deposition rate.

1-7. (Canceled)
 8. A silicon dioxide film forming method comprising: areaction chamber heating step of heating a reaction chamber to apredetermined temperature, the reaction chamber containing an object tobe processed having a surface provided with at least a silicon layer; agas pretreating step of energizing a process gas to produce water, theprocess gas containing a compound gas including hydrogen and chlorine,and oxygen gas; and a film forming step of forming a silicon dioxidefilm by supplying the process gas that has been energized to producewater into the heated reaction chamber to oxidize the silicon layer ofthe object to be processed.
 9. A silicon dioxide film forming methodaccording to claim 8, wherein the water is produced in the gaspretreating step to an extent such that the process gas does not producewater any further at the temperature to which the reaction chamber isheated.
 10. A silicon dioxide film forming method according to claim 8,wherein the process gas is energized to produce water by heating theprocess gas, in the gas pretreating step.
 11. A silicon dioxide filmforming method according to claim 10, wherein the process gas is heatedto a temperature that is higher than the temperature at which thereaction chamber is heated in the reaction chamber heating step.
 12. Asilicon dioxide film forming method according to claim 8, wherein thecompound gas including hydrogen and chlorine is a hydrogen chloride gas.13-31. (Canceled)